Many telephone company central offices (CO) transmit a stutter dial tone (SDT) signal on a telephone line to indicate to customer premises equipment (CPE) that a particular condition exists. For example, an SDT signal is used to indicate that a voice message exists in a voice mailbox associated with the telephone line.
In an industry as highly standardized as the telephone industry, the specification for the SDT signal is quite loose and it can be difficult build a device that will detect all SDT signals used throughout the United States, or more widely. An SDT signal is defined by the American National Standards Institute (ANSI) to be a periodic signal having component frequencies of 350 Hz and 440 Hz and which is alternately applied for no less than 100 ms and then suppressed for no less than 100 ms. Frequencies and pulsing durations vary widely within these specifications, and therefore any device for detecting an SDT signal must be both broadly tolerant and yet sufficiently discriminating. Such detection devices are generally tested under a wide range of conditions during their design phases so that the appropriate parameters can be selected empirically to reach the best practical compromise.
Conventionally, robustly detecting an SDT signal has proven difficult. Generally, detection has been done by estimating the electrical energy on the telephone line by measuring the envelope of either the line current or voltage and comparing the result to an expected value. The envelope, rather than the raw line current or voltage, is measured because an SDT signal is an AC signal. Moreover, the DC level of the line current or voltage changes when more than one CPE samples a telephone line in search of an SDT signal.
Detecting an SDT signal presents two main challenges. Firstly, one must detect signal transitions, that is, whether the signal has crossed a threshold from an off-state to an on-state or an on-state to an off-state. Secondly, one must determine whether a detected threshold crossing is a component of an SDT signal or noise.
As previously mentioned, detector parameters such as the threshold level are conventionally selected as an empirical compromise after much experimentation. However, such predetermined compromises will not likely be optimal for any particular line condition or SDT signal. What would be desirable is a method for detecting an SDT signal wherein the detection parameters are adapted to the particular characteristics of the relevant telephone line and the relevant SDT signal being sought.